Boot from ISO image: what "root" device?

[Pinna Marco]

Hì
I'm tryin' to boot an ubuntu live ISO image from my hd but the boot process stops when it try to mount the root device...

this is the code i use in grub.cfg (using grub2)

menuentry "Ubuntu, Live ISO Image" {
    linux    (hd0,4)/Uby/vmlinuz isofrom=(hd0,4)/Uby/Uby904.iso vga=791
    initrd   (hd0,4)/Uby/initrd.gz
}

it looks like i have to pass the "root" option in the "linux" line but i don't know what it should point to
(the iso image itself?)
:D

(hd0,4) is an ext3 filesystem (usually mounted as /home)


after booting starts it stops with the message

Waiting for root file system....
..........

it says that root filesystem does not exist and then it opens a shell.

i extracted the "vmlinz" and "initrd.gz" files from the iso i'm tryin to boot on

I think i'm doing somenthing wrong, but i cant find any useful documentation to solve the problem by myself
why the manual documentation is so poor?

regards, Marco